Method for depositing crystalline titania nanoparticles and films

ABSTRACT

The present invention provides a one-step and room-temperature process for depositing nanoparticles or nanocomposite (nanoparticle-assembled) films of crystalline titanium dioxide (TiO 2 ) onto a substrate surface using ultrafast pulsed laser ablation of Titania or metal titanium target. The system includes a pulsed laser with a pulse duration ranging from a few femtoseconds to a few tens of picoseconds, an optical setup for processing the laser beam such that the beam is focused onto the target surface with an appropriate average energy density and an appropriate energy density distribution, and a vacuum chamber in which the target and the substrate are installed and background gases and their pressures are appropriately adjusted.

The present application claims priority from provisional application60/899,892, the content of which is expressly incorporated by referenceherein.

FIELD OF THE INVENTION

This invention is related to a low-temperature process of depositingnanoparticles and nanocomposite films of crystalline titanium dioxide(TiO₂) onto a substrate surface using ultrafast pulsed laser ablation.

DESCRIPTION OF THE PRIOR ART

TiO₂ is a multi-functional material that has attracted extensiveresearch and development efforts in the last two decades. Newapplications in energy and environmental fields are being pursued, inaddition to its traditional usage as a white pigment. The newapplications of TiO₂ include gas sensors, electrochromic devices,dye-sensitized solar cells, and photocatalysts. Various photocatalystshave been developed using TiO₂ and have applied to fields such asair/water purification, self-cleaning, anti-fogging(hydrophilic/hydrophobic switching), sterilization, and hydrogenproduction through water-splitting. Two critical properties of TiO₂ thatdetermine its application are the crystal structure and surfacemorphology. Usually, a “nanocrystalline” structure is ideal for TiO₂films to achieve high functional performance. This is because i) thehigh specific surface area provides superior surface activity when theparticles (or grains) are of nanoscale dimensions; and ii) catalyticactivity is sensitively associated with the crystallinity of individualnanoparticles, and good crystallinity (in anatase, brookite or rutilestuctures) is desired.

With the rapid growth and expansion of TiO₂ applications, there is anemerging demand for large area deposition of crystalline TiO₂ onsubstrates such as plastics, polymer films, and glasses. Thesesubstrates are unstable under heating, and it is also very difficult toheat them uniformly over a large area. A desirable deposition process isneeded to permit fabrication of crystalline TiO₂ films below 300° C. oreven without any heat treatment.

Nanoparticles and nanocrystalline films of TiO₂ have been conventionallyproduced by various techniques including “wet processes”, such assol-gel deposition, i.e., aqueous precipitation reactions followed bypost-annealing treatment; and “dry processes”, such as physical vapordeposition (PVD), chemical vapor deposition (CVD), and sputtering. Inthe physical methods, elevated temperatures above 300° C. (up to 550°C.) are normally used during deposition and/or post-annealing to realizecrystalline TiO₂ films. In addition to the difficulties in heating,there is also a challenge for the conventional techniques to achievedesired morphologies and nanostructures while maintaining purity,stoichiometry, and homogeneity of TiO₂. Schichtel (U.S. Pat. No.7,135,206) teaches a chemical (wet) process to coat TiO₂ nanoparticlesby using oxide, hydroxide and urease/urea as the enzymatic precipitantsystem.

Recently, a technique of pulsed magnetron sputtering (PMS) has beenemployed to deposit crystalline TiO₂ films at room-temperature (See,“Photocatalytic titanium dioxide thin films prepared by reactive pulsemagnetron sputtering at low temperature”, D. Glöβ, P. Frach, O.Zywitzki, T. Modes, S. Klinkenberg, C. Gottfried, Surface CoatingsTechnology, 200, 967-971, 2005, and. “Deposition of photocatalytic TiO₂layers by pulse magnetron sputtering and by plasma-activatedevaporation”, Vacuum, 80, 679-683, 2006. See also “Crystallized TiO₂film growth on unheated substrates by pulse-powered magnetronsputtering”, M. Kamei, T. Ishigaki, Thin Solid Films, 515, 627-630,2006). This process nominally works without heating, however thesubstrate temperature is actually raised by plasma bombardment (Ref. 3).In general, a certain level of heating is necessary for makingcrystalline TiO₂. For example, J. Musil et al. reported a minimumsubstrate temperature of 160° C. to form crystalline TiO₂ films withreactive magnetron sputtering (See, “Low-temperature sputtering ofcrystalline TiO₂ films”, J. Musil, D He{hacek over (r)}man, and J.{hacek over (S)}icha, J. Vac. Sci. Technol. A 24, p. 521, 2006).

Pulsed laser deposition (PLD), also known as pulsed laser ablation(PLA), has appeared to be another promising technique to fabricatenanoparticles and films of crystalline TiO₂ at low temperatures. (See,“Preparation of TiO₂ nanoparticles by pulsed laser ablation: ambientpressure dependence of crystallization”, Jpn. J. Appl. Phys. 42,L479-481, 2003 and “Preparation of nanocrystalline titania films bypulsed laser deposition at room temperature”, N. Koshizaki, A. Narazaki,T. Sasaki, Applied Surface Science, 197-198, 624-627, 2002). Aparticular advantage of this technique is the highly energetic plasmagenerated by laser ablation.

Conventional PLD/PLA methods mostly employ nanosecond pulsed lasers suchas Q-switched excimer lasers and Nd:YAG lasers. The intense laserirradiation heats the material surface, and leads to surface melting andvaporization. At sufficient irradiance, the vapor can become ionized,and a plasma is formed (which is called plume). Nanocrystalline TiO₂particles can then be generated through forced condensation of theablated vapor in a high pressure (>1 Torr) background gas. In thenanosecond PLD/PLA approach, the resultant nanoparticles often have awide size distribution ranging from a few nanometers to a few hundredsof nanometers. The major drawbacks of this technique include unavoidableformation of very large (micron-sized) droplets from the splashing ofmolten target and difficulty in large-area deposition.

J. M. Lackner proposed a scale-up solution for industrial use of the PLDtechnique in depositing Ti-base films, in which a multi-beam scheme ofhigh power Nd:YAG laser light was used to coat the films at roomtemperature. (See, “Industrially-styled room temperature pulsed laserdeposition of titanium-based coatings”, J. M. Lackner, Vacuum, 78,73-82, 2005). However, the controllability of the crystallinity of TiO₂is still a challenge.

A few PLD/PLA related prior patents include: JP 2002-206164, whichdiscloses a double-beam method in nanosecond PLD to deposit crystallineTiO₂ film at elevated temperatures (>600° C.); JP 2002-20199, whichdiscloses a nanosecond PLD to grow epitaxial rutile-TiO₂ films; and JP2004-256859, which discloses a PLD method to produce amorphous TiO₂nanoparticles.

With the commercial availability of ultrafast pulsed lasers with typicalpulse durations ranging from a few femtoseconds to tens of picoseconds,ultrafast PLA/PLD has attracted much attention. Due to the extremelyshort pulse duration and the resultant high peak power density, theablation threshold is reduced by 1-2 orders of magnitude compared withnanosecond PLA, and as a result, the commonly favored ultravioletwavelength (which is expensive to obtain) in nanosecond PLA is no longera requirement in ultrafast PLA. A prior patent (US RE 37,585) provides aguideline to realize efficient laser ablation by selecting appropriatepulse duration and taking advantage of the low ablation thresholds.

A few theoretical and experimental studies have suggested that ultrafastPLA also generates nanoparticles, but with a fundamentally differentmechanism from those processes using longer (nanosecond) pulses. (See,A. V. Bulgakov, I. Ozerov, and W. Marine, Thin Solid Films 453, p. 557,2004, and S. Eliezer, N. Eliaz, E. Grossman, D. Fisher, I. Couzman, Z.Henis, S. Pecker, Y. Horovitz, M. Fraenkel, S. Maman, and Y. Lereah,Physical Review B, 69, p. 144119, 2004. See also, S. Amoruso, R.Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, G. Ausanio, V.Iannotti, and Lanotte, Applied Physics Letters, 84, No. 22, p. 4502,2004). In ultrafast PLA, nanoparticles are generated automatically as aresult of phase transition near the critical point of the material underirradiation, which is only reachable through ultrafast heating. Also,unlike the forced condensation/nucleation process in nanosecond PLA,which occurs long after the ablation is over, the nanoparticlegeneration in ultrafast pulsed laser ablation takes place at a veryearly stage during ablation (less than one nanosecond after the laserpulse hits the target), and the energetic nanoparticles fly out in avery directional manner. These features in principle should enable aone-step process that can cover both the particle generation anddeposition. Thus, both nanoparticles and nanocomposite films, i.e.,nanoparticle-assembled films can be deposited onto substrates with goodadhesion (due to the kinetic energy of the particles) by using theultrafast PLA method.

Based on the inventors' previous systematic investigation, a patentapplication (U.S. Provisional Application 60/818289, incorporated byreference herein) and a publication were disclosed recently, in whichthe experimental parameters for size selection and crystallinity control(at room temperature) were described for nanoparticle generation usingan ultrafast PLA process. (See, B. Liu, Z. Hu, Y. Chen, X. Pan, and Y.Che, Applied Physics Letters, 90, p. 044103, 2007). The presentinvention is an application of the above process specifically used formetal oxides, such as TiO₂, and provides a one-step process to depositnanoparticles and nanocomposite (nanoparticle-assembled) films ofcrystalline TiO₂ at room-temperature.

All patent applications, patents and publications referred to above areexpressly incorporated by reference herein, and particularly, thefollowing patent documents disclosing related subject matter asreferenced above are incorporated by reference herein: US RE 37,585 toMourou et al.; JP 2002-206164 to Sasaki et al.; JP 2002-20199 to Yamakiet al. ; JP 2004-256859 to Sai et al.; 60/818,289 to Liu et al.; andU.S. Pat. No. 7,135,206 to Schichtel.

SUMMARY OF THE INVENTION

This invention is related to producing nanoparticles and nanocompositefilms of crystalline TiO₂ by using ultrafast pulsed laser ablation(PLA). Instead of heating up the substrate during deposition orpost-annealing after deposition, the crystalline TiO₂ is realized atroom temperature. This room-temperature process, which benefits from theultrafast PLA as described in the last section, enables coating offunctional TiO₂ particles or films onto heat-sensitive materials such asglasses, plastics, papers, and polymer films.

The nanoparticles or the grains of nanocomposite films haveparticle/grain size ranges from a few nanometers up to one micron. Theparticle/grain size is controllable mainly by selecting an appropriatelaser fluence (see, U.S. 60/818,289).

The nanocomposite TiO₂ films can be nanoparticle-assembled filmsproduced by continuous deposition of TiO₂ nanoparticles; or can be acomposite with a host film embedded with nanoparticles of crystallineTiO₂. The host film can be of titanium oxides (TiO_(X)) in eithercrystalline or amorphous forms. It can also be any other host materialsuch as ceramics or polymers. The nanocomposite films can be produced byalternately or simultaneously depositing the host material andcrystalline TiO₂ nanoparticles. A variety of material combinations canbe easily realized by alternating targets of different materials insidethe deposition chamber.

TiO₂ has three major crystalline structures, including rutile, anatase,and brookite. Rutile is known as the thermally stable (high temperature)phase produced normally under a temperature higher than 500° C.; andanatase and brookite are metastable (low temperature) phases which canbe transformed to rutile under high temperature. It is known thatanatase usually exhibits better photocatalytic activities. However insome applications, rutile is preferred due to its slightly narrower bandgap of 3.0 eV (compared to 3.2 eV for anatase), and higher dielectricconstant. In the ultrafast PLA process, the crystal structures of TiO₂are no longer determined by the temperature of the substrate or theannealing process. They are mainly controlled by laser parameters, suchas laser fluence (or pulse energy), pulse width, repetition rate, andwavelength. For the present invention, anatase and rutile or theirmixtures are preferred; and a pulse width of 10 fs-100 ps, a laserfluence of 10 mJ/cm²-100 J/cm² (pulse energy of 100 nJ-10 mJ), and arepetition rate of 1 kHz-100 MHz are preferred. Because laser fluence isa critical parameter in ultrafast PLA, this invention also employs anoptical setup to transform the laser beam from a Gaussian profile to a“flat-top” profile to realize a uniform fluence on the target surface.

In addition to the above laser parameters, the background gas(es) andtheir pressures also provide additional control over the crystallinity,stoichiometry, and the morphology of particles and films. In theultrafast PLA process, desired crystallinity, stoichiometry, and themorphology of TiO₂ can be realized either by ablating a titanium oxide(TiO_(x)) target or metal titanium target in a background gas of oxygenor a gas mixture containing oxygen with appropriate partial and totalpressures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the set up of the pulsed laser deposition system. Thesystem includes a vacuum chamber (and related pumps, not shown in thefigure), a target manipulator, an ion probe (Langmuir probe), a gasinlet, and a substrate manipulator. The laser beam is focused onto thetarget surface through a fused silica window.

FIG. 2A shows the X-ray diffraction pattern of a sample deposited on aglass substrate. The sample was produced by ablating a titanium oxidetarget in vacuum with a laser fluence of 0.4 J/cm².

FIG. 2B shows the X-ray diffraction pattern of a sample deposited on asilicon wafer (100). The sample was produced by ablating a titaniumoxide target in vacuum with a laser fluence of 0.4 J/cm².

FIG. 2C shows the X-ray diffraction pattern of a sample deposited on aglass substrate. The sample was produced by ablating a titanium metaltarget under 100 Pa of oxygen with a laser fluence of 0.3 J/cm².

FIG. 3 is a transmission electron microscopy (TEM) image of a sampledeposited on a TEM grid. The sample was produced by ablating a titaniummetal target under 100 Pa of oxygen with a laser fluence of 0.4 J/cm².

FIG. 4 is a scanning electron microscopy (SEM) image of a sampledeposited on a silicon (100) wafer. The sample was produced by ablatinga titanium oxide target under 0.1 Pascal of oxygen with a laser fluenceof 0.4 J/cm².

FIG. 5A shows the selected area electron diffraction (SAED) pattern of asample deposited on a TEM grid. The sample was produced by ablating atitanium oxide target under 1.0 Pascal of oxygen with a laser fluence of0.6 J/cm². The pattern suggests the rutile structure.

FIG. 5B shows the SAED pattern of a sample deposited on a TEM grid. Thesample was produced by ablating a titanium metal target under 100 Pa ofoxygen at a laser fluence of 0.4 J/cm². The pattern suggests theexistence of both the rutile and anatase structures.

FIG. 5C is a high resolution TEM image of a sample deposited on a TEMgrid. The sample was produced by ablating a titanium oxide target under300 Pa of oxygen with a laser fluence of 0.4 J/cm². The crystal wasviewed with the electron beam aligned to its [001] direction. Clearlattice fringes are evident, indicating good (single) crystallinity ofthe particle.

FIG. 5D is a high resolution TEM image of a sample deposited on TEMgrid. The sample was produced by ablating a titanium oxide target invacuum at a fluence of 0.2 J/cm². The lattice fringes in the image are<101> planes of a brookite crystal and the crystal was viewed down inits [010] direction.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a one-step room-temperature process fordepositing nanoparticles and nanocomposite (i.e.,nanoparticle-assembled) films of crystalline titanium dioxide (TiO₂)onto a substrate surface using ultrafast pulsed laser ablation ofTitania or metal titanium targets.

FIG. 1 illustrates the experimental system used in this invention. Thesystem includes a vacuum chamber pumped by a turbo pump and a mechanicalpump, a target manipulator which provides rotational and lateralmovements for four targets of different materials, a substratemanipulator which provides heating and rotational and lateral movementsfor the substrate, a gas inlet through which reactive gases are providedand their pressures are appropriately adjusted, and an ion probe(Langmuir probe) to measure the ion current of the ablation plume, whichis used as an indicator for focusing the laser beam on the targetsurface. When measuring the ion current, the ion probe is biased −50 Vrelative to ground to collect the positive ions in the plume (the numberof negative ions in plasma is negligible). An ultrafast laser (not shownin the figure) is positioned outside the chamber and the laser beam isfocused onto the target surface through a fused silica window. The laserhas a pulse width between 10 fs-50 ps, preferably between 10 fs-1 ps; apulse energy between 100 nJ-10 mJ; and a repetition rate greater than 1kHz. Particularly, a femtosecond pulsed fiber laser system (FCPA μJewelD-400, IMRA America, Inc., which emits a laser beam of pulses with pulseduration in the range of 200-500 fs, a wavelength of 1045 nm and arepetition rate between 100 kHz and 5 MHz) was used in this study.

The system also includes an optical setup for processing the laser beamsuch that the beam is focused onto the target surface with anappropriate average energy density and an appropriate energy densitydistribution.

The targets used in this invention are round disks of titanium metal andcompressed TiO₂ powder. The packing density of the target is expected tobe as high as possible (at least >50% of its theoretical density). Theshape of the target is not limited to a round disk. It can be square ora rectangular pellet, or of arbritary shape. However, the target needsto have at least one smooth surface for laser ablation.

Titanium metal and titanium oxide TiO₂ are used as example materials inthis invention, but this invention is not limited to these materials,because the physics and chemistry behind crystalline titania formationduring ultrafast pulsed laser ablation applies similarly to othercompounds containing elemental titanium, as long as the materialcontains elemental titanium that can react with oxygen during laserablation to form TiO₂.

In this invention, an ultrafast pulsed laser beam is focused onto atarget. A plume of plasma (containing ions, neutrals, and smallparticles) is emitted and collected by a substrate to form the film. Thesubstrate can be made of any material sustainable in a vacuum. This isbecause the film deposition is processed at the ambient (room)temperature and there is no substrate heating requirement in thisinvention.

FIGS. 2A-2C display the X-ray diffraction (XRD) θ-2θ patterns of thetitania films deposited on glass and silicon substrates under variousconditions. The standard powder XRD pattern of rutile (from the PowderDiffraction File published by the International Center for DiffractionData) is also indicated in the lower half of each figure. The XRDresults of FIG. 2A and FIG. 2C are recorded with a Rigaku MiniFlex X-RayDiffractometer. The XRD pattern in FIG. 2B is recorded with a RigakuRotoflex Diffractometer that has a higher X-ray intensity and a bettersignal to noise ratio on the diffraction patterns. In particular, FIG.2A shows the XRD result of the crystalline structure of a sampledeposited on a glass substrate by ablating a titanium oxide target invacuum with a laser fluence of 0.4 J/cm². The X-ray diffraction patternof the sample in FIG. 2A possess several diffraction peaks whichcorrespond to the rutile phase. FIG. 2B shows the XRD result of thecrystalline structure of a sample deposited on a single crystal silicon(100) substrate by ablating a titanium oxide target in vacuum with alaser fluence of 0.4 J/cm². This XRD pattern possesses severaldiffraction peaks which correspond to the rutile phase. The strongestpeak (at 2θ=33°) comes from the silicon substrate. FIG. 2C shows the XRDresult of the crystalline structure of a sample deposited on a glasssubstrate by ablating a titanium metal target under an oxygen pressureof 100 Pa with a laser fluence of 0.3 J/cm². This XRD patterns alsocorrespond to the rutile phase. This suggests that the ablated titaniummetal reacts with oxygen to form titanium oxide. It is worth noting thatonly the rutile phase is exhibited in the above XRD results.

The morphologies of the deposited titania were analyzed using SEM andTEM. FIG. 3 displays an SEM image of a film deposited on a siliconsubstrate by ablating a titanium oxide target under 0.1 Pa of oxygen. Itis evident that this film is mostly composed of small particles withsizes less than one micrometer. The larger particles are aggregates ofthe smaller nanoparticles. The TEM image in FIG. 4 further confirms thisobservation, which shows the particle size is less than 100 nanometers.(The sample for FIG. 4 was deposited on a TEM grid by ablating atitanium metal target under an oxygen pressure of 100 Pa with a laserfluence of 0.4 J/cm².)

Select Area Electron Diffraction (SAED) is used to analyze in moredetail the crystal structure of the deposited titania particles. FIGS.5A and 5B display the SAED patterns of two samples deposited on TEMgrids. The SAED pattern in FIG. 5A can be indexed to the rutilestructure. The SAED patterns in FIG. 5B show the coexistence of rutileand anatase phases. The lattice fringes in the high resolution TEM(HRTEM) image (FIG. 5C) are obtained by an electron beam aligned to theTiO₂ [001] direction, and the lattice fringes in the HRTEM image (FIG.5D) are obtained by an electron beam aligned to the brookite TiO₂ [010]direction. The well parallel fringes indicate a single crystallineparticle.

Although a few exemplary embodiments of the present invention have beenshown and described, the present invention is not limited to thedescribed exemplary embodiments. Instead, it will be appreciated bythose skilled in the art that changes may be made to these exemplaryembodiments without departing from the principles and spirit of theinvention, the scope of which is defined by the claimed elements andtheir equivalents.

1. A method for depositing nanoparticles or nanocomposite films ofcrystalline TiO₂, comprising: providing a target comprised of atitanium-containing material; providing a substrate to support thedeposited particles or films; and ablating regions of said target withultrafast laser pulses to create a plume of particles directed towardsaid substrate.
 2. The method of claim 1, wherein the said nanoparticleshave sizes of less than 1 micron.
 3. The method of claim 1, wherein thesaid nanocomposite films are films assembled of nanoparticles ofcrystalline TiO₂.
 4. The method of claim 1, wherein the saidnanocomposite films are composed of a host material embedded withnanoparticles of crystalline TiO₂.
 5. The method of claim 1, wherein thesaid crystalline TiO₂ is in anatase phase or rutile phase or brookitephase or a mixture of any two or all three phases.
 6. The method ofclaim 1, wherein said target includes elemental titanium, and saidablation takes place in an oxygenated atmosphere so that saidcrystalline TiO₂ is formed subsequent to said ablation.
 7. The method ofclaim 1, further comprising the steps of: providing a vacuum chambercontaining said target and said substrate, and wherein said ablationstep comprises irradiating the target with a laser beam generated by anultrafast pulsed laser, the said laser beam being processed and focusedonto the target by an optical system.
 8. The method of claim 1 or 7,wherein the said deposition is performed at a substrate temperaturelower than 300° C.
 9. The method of claim 8, wherein the said depositionis performed at room temperature.
 10. The method of claim 1 or 7,wherein the said substrate is a heat sensitive material, including oneof glass, paper, plastic and polymer.
 11. The method of claim 1 or 7,wherein the ultrafast pulses have a pulse width of 10 fs-100 ps.
 12. Themethod of claim 1 or 7, wherein the ultrafast pulses each have a pulseenergy of 100 nJ-10 mJ.
 13. The method of claim 7, wherein the ultrafastpulsed laser has a repetition rate of 1 kHz-100 MHz.
 14. The method ofclaim 7, wherein the ultrafast pulsed laser and the optical systemenable a laser fluence in the range of 10 mJ/cm²-100 J/cm², at thetarget surface.
 15. The method of claim 7, wherein the optical systemprocesses the intensity distribution of the laser beam from a Gaussianprofile to a ‘flat-top’ profile.
 16. The method of claim 1 or 7, whereinthe said target is a metal containing titanium or an oxide containingtitanium oxide.
 17. The method of claim 1 or 7, wherein said targetincludes titanium oxide and deposition onto said substrate takes placein vacuum or in background gas(es) containing oxygen.
 18. The method ofclaim 7, wherein said target includes elemental titanium and the step ofperforming laser ablation takes place within background gas(es)containing oxygen.